Generally with a conventional inkjet printing method using an aqueous inkjet ink, printing is performed on a bond paper or other normal type of paper or on a specialized paper, typically designed to have a high ink absorbing property, etc. However, offset coated media that are suitable for offset printing are not suited for printing using an aqueous inkjet ink, because a substrate surface is coated with a smooth, hydrophobic, and non-porous coating.
Even if printing using an aqueous inkjet ink is performed, because the coating does not absorb the aqueous inkjet ink, the inkjet ink is repelled by or bleeds on the coating surface and a clear print with sharp edges cannot be obtained. Further, because the print is poor in drying property and inadequate in fixing property even upon drying, bleeding occurs readily when the print is rubbed. Development of a new art for performing printing satisfactorily on offset coated media by an inkjet printing method is thus being demanded.
For performing printing satisfactorily on offset coated media by an inkjet printing method, improvement of the affinity of an inkjet ink to a hydrophobic coating is considered first, and for this purpose, surfactant types have been examined and blending of glycol ether or a wetting agent, etc., that exhibits compatibility with water and yet exhibits affinity with the hydrophobic coating has been proposed. For improvement of the fixing property as well as improvement of the affinity of the printed inkjet ink on the coating, blending of a water-soluble or water-dispersible binder resin (vehicle) has also been proposed (see, for example, Patent Documents 1 and 2, indicated below).
Combined use of water and a solvent, having compatibility with water and exhibiting permeability to a hydrophobic coating, is also considered, and the use of a polar aprotic solvent as such a solvent has been proposed (Patent Document 3).    Patent Document 1: Japanese Unexamined Patent Publication No. 2003-206426 (Claims and Paragraphs 0009 to 0010)    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-510028 (Claims and Paragraph 0012)    Patent Document 3: Japanese Unexampled Patent Publication No. 2003-268279 (Claims and Paragraph 0009)